Editing Open-chain compound (section)

{{Short description|Type of organic molecule with a linear structure}}
In [[chemistry]], an '''open-chain compound''' (or '''open chain compound''') or '''acyclic compound''' (Greek prefix ''α'' 'without' and ''κύκλος'' 'cycle') is a compound with a linear structure, rather than a [[Cyclic compound|cyclic]] one.<ref>{{GoldBookRef|file=P04724|title=polycyclic system}}</ref>
An open-chain compound having no [[side group]]s is called a '''straight-chain compound''' (also spelled as straight chain compound).<ref>{{cite journal|last1=Coles|first1=Lydia|title=A chromatographic method for the separation of branched-chain and straight-chain compounds of columns containing urea|journal=Journal of Chromatography A|volume=32|pages=657–661|year=1968|issue=4|doi=10.1016/S0021-9673(01)80544-6|pmid=5645558}}</ref><ref>{{GoldBookRef|title=silazanes|file=S05669}}</ref> Many of the simple molecules of [[organic chemistry]], such as the [[alkane]]s and [[alkene]]s, have both linear and ring [[isomer]]s, that is, both acyclic and [[cyclic compound|cyclic]]. For those with 4 or more carbons, the linear forms can have straight-chain or branched-chain isomers. The lowercase prefix ''n-'' denotes the straight-chain isomer; for example, ''n''-butane is straight-chain [[butane]], whereas ''i''-butane is [[isobutane]]. [[Cycloalkane]]s are isomers of alkenes, not of alkanes, because the ring's closure involves a C-C bond. Having no rings (aromatic or otherwise), all open-chain compounds are [[aliphatic compound|aliphatic]].

Typically in [[biochemistry]], some isomers are more prevalent than others. For example, in living organisms, the [[glucose#Open-chain form|open-chain isomer of glucose]] usually exists only transiently, in small amounts; [[glucose|D-glucose]] is the usual isomer; and [[L-Glucose|L-glucose]] is rare.

Straight-chain molecules are often not literally straight, in the sense that their [[bond angle]]s are often not 180°, but the name reflects that they are [[schematic]]ally straight. For example, the straight-chain alkanes are wavy or "puckered", as the models below show.

{| class="wikitable"
|-align="center"
| [[Image:Isopentane-2D-skeletal.svg|110px]]
| [[Image:Pentane-2D-Skeletal.svg|150px]]
| [[File:Cyclopentane v2.svg|50px]]
|-align="center"
| branched-chain
| straight-chain
| cyclic
|-align="center"
| colspan=2 | open-chain
|}

[[File:Nonane 3D ball.png|thumb|left|''n''-Nonane]]

[[File:HexosesHemiacetal.png|thumb|left|upright=2.5|The top row shows the open-chain forms of [[glucose]] and [[mannose]]. The lower row shows the cyclic forms.]]
{{clear}}